1. Field of the Invention
The present invention relates to the field of cables. More particularly, the present invention relates to filler components used in communication cables.
2. Description of Related Art
Communication cables are broadly grouped into two arrangements, fiber optic cables and metal conductor cables, each of which has their own unique set of construction parameters that affect the quality of the communication signals carried therethrough.
Regarding metal conductor cables, one typical arrangement is the LAN (Local Area Network) cable that is usually constructed of four pairs of twisted insulated copper conductors encased within a jacket. Other larger cables may employ more pairs of conductors.
In this typical four pair LAN cable construction, in addition to protecting against external environmental interferences, in order to decrease cross talk between signals passing through one pair, and signals passing through adjacent pairs within the same LAN cable, the pairs of conductors are twisted. Moreover, as the signal interference between pairs is highest when conductors of adjacent pairs lie parallel to one another, pairs are twisted around one another at different rates (i.e. at different lay lengths) to minimize the instances of parallel conductors in adjacent pairs. Other items such as tapes, fillers, or cross fillers may be added to even further reduce the amount of cross talk between pairs within the cable.
For example, in prior art arrangements where four twisted pairs are included in one jacket it is common to use four different lay lengths, one for each of the four twisted pairs. These varied rates of twisting result in a reduced number of incidences where the wires in the pairs run parallel to one another, effecting a reduction in crosstalk. For example, in a typical four pair cable, arranged in a compact square/rectangle, there are six different crosstalk combinations that need to be addressed, as shown in prior art FIG. 1 (labeled C1-C6).
It is typically known that the shorter the lay length of a particular pair in a multi-pair cable, the more crosstalk is reduced. However, shorter lay lengths obviously use more wire per length of cable, and thus there are limitations on how short the lay length can be in any given copper wire twisted pair. Therefore, it is ideal to have the longest lay length possible that meets the desired crosstalk threshold.
One prior art manner for addressing such cross talk issues is to isolate the longest lay length pair in a four pair LAN cable, making it equidistant to the other three pairs in the same cable and as far as possible from other pairs in adjacent LAN cables. For example, as shown in U.S. Pat. No. 7,550,674, a plurality of unshielded twisted pairs are provided, each of which has a different lay length. The jacket encloses the plurality of unshielded twisted pairs, and the unshielded twisted pair that has the longest lay length among the plurality of unshielded twisted pairs positioned within the center of the jacket, substantially along the central longitudinal axis of the cable. See prior art FIG. 2.
To maintain such geometry and its advantageous electrical characteristics, bumper elements are disposed around the central pair in between the outside pairs. The bumper elements are typically polymers formed as solid, foamed or hollow structures, however, alternative materials and structures may be used. These bumpers are advantageously of a dimension substantially equal to the diameter of a twisted pair, and are used for maintaining a regular geometry along the length of cable as shown in FIG. 2.
However, the necessity of the bumpers to maintain the pair geometry in the cable necessarily leads to the drawback of using additional components in the cable, which is always a disadvantage in cable construction owing to added size, weight, cost and fuel load (fuel load affects the flame and smoke performance of cable constructions in flame tests).
Another problem with these bumpers is that their proximity to the pairs that they separate disturbs the signal's electromagnetic field and reduces the effectiveness of the transmission signal through the pair owing to the detrimental dielectric properties of the polymers from which they are constructed. Although foaming the polymers used to make these bumpers is a possible solution and in theory could yield improved electrical performance, foaming is generally a non-preferred option owing to its added processing/extrusion difficulties versus solid profile extrusion.